Plant Physiol. (1989) 90, 907-9120032-0889/89/90/0907/06/$01 .00/0

Received for publication September 20, 1988and in revised form January 29, 1989

Hormone Levels and Apical Dominance in the Aquatic FernMarsilea drummondii A. Br.

Gilles Pilate, Lucienne Sossountzov*, and Emile MiginiacLaboratoire de Physiologie veg6tale, CNRS (UA 1180), T 53, 5eme etage, Universit6 P. et M. Curie, 4 Place

Jussieu, 75252 Paris Cedex 05, France

ABSTRACT

Terminal buds and successively subjacent lateral buds of thewater fem, Marsilea drummondii, were examined to determinethe pattem of hormone distribution in relation to apical domi-nance. Quantitative levels of indole-3-acetic acid (IAA), abscisicacid (ABA), zeatin and zeatin riboside (Z and ZR), and isopenten-yladenosine (iPA) were determined by a solid-phase immunoas-say using polyclonal antihormone antibodies. Enzyme-linked im-munosorbent assay was used following a one-step HPLC purifi-cation procedure to obtain the free hormones. Active shoot apicescontained the most IAA and Z-type cytokinins and inhibited budsthe least. No significant differences in ABA levels were foundleading to the conclusion that ABA did not play any role in apicaldominance. The normal precedence of the most rapid outgrowthof the youngest inhibited bud as observed previously in decapi-tated plants was well correlated with its very high level of iPAobserved in this study. The same phenomenon was observed inthe median buds but with a weaker amplitude. The presence ofthis storage form could indicate that a bud at its entry intoquiescence eventually looses the ability to hydroxylate iPA to Z-type cytokinins when it is fully inhibited. IAA and Z + ZR areconcluded to be essential for lateral bud growth.

The lateral buds of the water fern, Marsilea drummondii,are quiescent due to the inhibition produced by the mainshoot tip and its young leafprimordia (1). Over the last severalyears, we have characterized all these buds in intact anddecapitated plants in an attempt to elucidate the reasons forthe inhibitory role of the terminal bud (2, 3, 11-16). We haveshown that release of lateral buds from apical dominance canbe achieved by various means: surgical removal of the shoottip, or, in the presence of the terminal bud, application ofsynthetic (kinetin) or naturally occurring (auxin) hormones,little prickings at the bud base (2). But the removal or prickingtype experiments were difficult to interpret due to the acti-vation effects ofwounding. Studies with exogenous hormones,although widely used, have not furthered our understandingof their involvement in these regulatory processes. Auxin toreplace totally or partially the main shoot tip or cytokinins torelease the axillary buds from the apical inhibition, must beapplied directly at very high levels. All these experiments ledto the hypothesis that inhibited buds under-produced IAAand/or CKs.' We then employed several other approaches

' Abbreviations: CKs, cytokinins; DW, dry weight; FW, freshweight; iP, isopentenyladenine; iPA, isopentenyladenosine; Z, zeatin;ZR, zeatin riboside.

including: movements into the buds of nutriments, ions (16),hormones (3, 13), cytochemical characterization (ATPases[ 15], DNA and RNA synthesis [ 12]). They have demonstratedthe sink role of the terminal bud and the privileged status ofthe youngest subapical bud: the last bud to enter in quiescenceis the first to resume growth after decapitation, although allthe older buds are also, but for a short while, able to begingrowing, according to an age gradient. These events may berelated to mobile hormones which are presumably synthetizedin the shoot tip and its young leaves or in the roots andtransported downward (auxin) or upward (CKs) in the stem.

Therefore, the possibility that endogenous hormone levelswere involved in the progressive inhibition of lateral buds wasstudied by comparing the IAA, CKs, and ABA contents ofthe active terminal bud and the inhibited buds inserted alongthe rhizome. The need for a simultaneous determination ofIAA, CKs, and ABA in the same material led us to useimmunoassays on HPLC- purified extracts from four types ofbuds from intact sporophytes differing by their age andactivity.

MATERIALS AND METHODS

Plant Material: Growth and Sampling Conditions

Marsilea drummondii sporophytes were axenically grownfrom cuttings at 23 ± 2°C in a glasshouse cabinet as previouslydescribed (12). On the 45th d of growth, terminal buds andlateral buds of successively increasing ages were excised undermagnifying glasses from 240 plants blotted with filter paperto remove the attached liquid medium. The buds, all removedbetween 9 and 10 AM, were immediately dropped in liquidnitrogen. After freezing, the plant material was quicklyweighted and freeze-dried for 48 h, and then accuratelyweighted again to obtain the DW. They were then stored at-40°C as a homogeneous powder until extracted for hormoneanalysis. The knowledge of the buds given by our previoushistological studies (1, 12) led us to take offequivalent tissues:we discarded the nodes for lateral buds and the long internodebelow the shoot tips.

Quantitative Determinations of Endogenous Hormones

The simultaneous extraction and purification ofIAA, ABA,ZR, and its corresponding base Z as well as iPA were per-formed according to the techniques described in detail earlier(8, 9, 20). Their levels were quantified using ELISA withantihormone antibodies. HPLC fractionation prior to ELISA

907https://plantphysiol.orgDownloaded on December 30, 2020. - Published by

Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

Plant Physiol. Vol. 90, 1989

test was used to remove interfering materials eventually pre-sent in the extracts and the compounds cross-reacting withthe polyclonal antibodies used. In short, freeze-dried pow-dered tissues were extracted with cold 80% methanol contain-ing an antioxidant for 60 h at 4TC in darkness. Extracts, withthe added tritiated standards (IAA and ABA from Amersham,UK, and iPA or ZR, gifts from Dr. Laloue, Gif-sur-Yvette,France) were filtered and then passed through a Sep Pak C 18cartridge (Waters Associates, MA). The resulting eluates werereduced to water in vacuo with a rotary evaporator. Afteracidification at pH 3, they were then submitted to a HPLCfractionation through a Licrospher column (Merck, FRG)fitted on a Beckman apparatus and eluted with a 3.3% aceticacid/methanol gradient. The fractions corresponding to thestandards were collected and reduced to dryness. The CKfractions were taken up with PBS without post-treatment andsubmitted to ELISA test with anti-ZR and with anti-iPAantibodies. The fractions containing IAA and ABA weremethylated with etheral diazomethane prior to ELISA quan-titation with anti-IAA and anti-cis-(+) ABA antibodies. Thelevel of each hormone in each sample was measured fivetimes, the values being corrected for recovery and diminishedof the corresponding amount of the recovered standards. Theresults were analyzed with calibration curves correspondingto the different fractions with an Apple MacIntosh computersystem. The results are expressed as the means ± SE on theDW and per organ basis. Because the anti-iPA antibodiescross-react with iP, the iPA contents correspond to iP + iPAnot separated during the HPLC purification.

RESULTS

As shown in Table I, the main shoot apex weighed aboutfive times more than the lateral buds on a FW basis and fourtimes on the DW basis. The arrest of growth and age of thelateral buds caused a significant loss of water content and arise in dry weight.

Comparative Hormone Levels in Active and InhibitedBuds

The most striking result was the finding that IAA, ABA,and CKs occurred in all the buds, in the active or the restingstate, with the exception of basal buds in which ZR was notdetected (Figs. 1-4).

Table I. Some Characteristics of the Buds of Marsilea drummondiiSubmitted to the Quantitation of Hormones

Nature of FW DW Waterthe bud Content

,ug/bud %TBa 2050 177 91.4SAB 336 31.3 90.7MB 392 41.7 89.4BB 394 43.8 88.9

a The values were obtained from 240 terminal buds (TB) or subap-ical (SAB), median (MB), and basal (BB) lateral buds.

IAA Contents

The highest concentration was found, as expected, in theterminal bud (Fig. 1), whatever the criteria used for calcula-tions. There was along the main axis an apico-basal gradientof the IAA contents corresponding to that of the age of thebuds and was considered the DW basis. But it is noteworthythat the youngest subapical bud which started to enter in aresting state contained about half of the IAA in the terminalbud. On per organ basis, lateral buds contained only 10% ofthe IAA present in the leading shoot apex. It is noticeable thatthe IAA levels dropped abruptly in the lateral buds; however,no distinct effect was found in relation with their relativeactivity nor their position on the main axis, i.e. their age.

ABA Contents

The distribution of ABA (Fig. 2) in the buds was ratherhomogeneous whatever the criteria used for calculations: thecorresponding differences were small when differences inactivity were high. The terminal bud grew well, despite anABA content equal to that found in the older buds. The factthat along the rhizome the maturation ofthe bud correspond-ing to a slight decrease in water content was not accompaniedby a rise in ABA is possibly to be connected with the aquatichabit ofM. drummondii, which can also explain its relativelylow concentrations in all buds. No concentration gradient

50 F-

25 1

0

1 0

5

0TB SAB MB 61

Figure 1. Comparative endogenous IAA levels in the active terminalbud (TB) and in the inhibited subapical, median, and basal lateralbuds (SAB, MB, and BB). Results are expressed as nmol/g DW andas pmol/organ, as mean of 5 replicates ±SE.

MEMOMML.,

908 PILATE ET AL.

https://plantphysiol.orgDownloaded on December 30, 2020. - Published by Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

ENDOGENOUS HORMONES AND APICAL DOMINANCE

500

250

0 L

20

1 0

0

--F- DW

l1r

TB SAB MB BBFigure 2. Comparative endogenous ABA levels in the active terminalbud (TB) and in the lateral inhibited buds (SAB, MB, and BB). Resultsare expressed as pmol/g DW and as fmol/organ, as mean of 5replicates ±SE.

was observed. The only noticeable result was the higherconcentration of ABA, on the basis of DW, in the subapicalbud, i.e. during the course of the process of its inhibition,while ABA level was equal to that measured in the terminalbud if expressed in terms of per bud.

CK Contents

Z contents, when expressed on a DW basis, decreased withbud age but peaked in the terminal and subapical buds (Fig.3). But, if the organ as a whole was taken as criterion forcalculations, there was a drastic drop of Z content in the notfully quiescent subapical bud when compared to the activeshoot tip, and an about 10-fold decrease in the older buds.The concentrations of ZR in the different buds were notrelevant to their relative activity. The contents were equivalentin the main shoot apex and in the quiescent median bud, butwere depressed in the subapical bud and were undetectable inthe basal buds. Here again, an analysis of the results on a perorgan basis was more correlated with the known state of thebuds: higher ZR content in the growing tips than in theinhibited buds.CK concentrations, expressed as the sum of Z + ZR, were

higher in the terminal bud, and a clear decrease with increas-ing bud age was observed. On the per organ basis, the twoCKs were also distributed as a function of bud age and clearlyaccording to a steep apico-basal gradient.

100

75

50

25

0

1 5

1 0

5

0

ORGAN

TB

E] ZR

* Z

SAB MB BBFigure 3. Comparative Z and ZR levels in the active terminal buds(TB) and in the lateral inhibited buds (SAB, MB, and BB). Levels areexpressed as pmol/g DW and as fmol/organ. The bars represent thesuperposed values of Z and ZR, as mean of five replicates +SE.

iPA Contents

The iPA contents, corresponding to iP + iPA as noted in"Materials and Methods," showed clearly, according to thetwo criteria used, the privileged nature of the subapical budand, to a lesser extent, of the median bud (Fig. 4): the formerwas by far richer in iPA content than the main shoot apex

and than the older sibling buds. It is noticeable that this bud,the first to resume growth in decapitated plants, contained avery high level of iPA whereas the amount in the terminalbuds was low.

Hormonal Balance

It is often assumed that hormonal balance, which supposesthat regulators are present simultaneously in the same organwith agonistic or antagonistic effects, is more important thanthe level of a single hormone to explain morphogeneticalprocesses. This is the reason we have calculated the IAA toABA and to Z + ZR ratios, eliminating the case of iPA withits very high value in the subapical bud. As shown in TableII, the most striking feature was the great difference betweenthe growing main apex and the inhibited buds when consid-ering the IAA/ABA ratio. All the buds contained about thesame level of ABA but differed in their IAA contents. Theactive bud contained more IAA as compared to ABA. Theratio was then about nine-fold less in the inhibited buds. Acomparatively low ratio was well correlated with the quies-

DW E] ZR

N Z

g -~~~Tm~~~~~~~~~~~ 909

https://plantphysiol.orgDownloaded on December 30, 2020. - Published by Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

Plant Physiol. Vol. 90, 1989

ORGAN

MB BBFigure 4. Comparative iPA levels in the active terminal buds (TB)and in the lateral inhibited buds (SAB, MB, and BB). Levels areexpressed as pmol/g DW and as fmol/organ, as mean of 5 replicates±SE.

Table II. Ratios of IAA to ABA and Z + ZRNature of IAA/ABAS IAA/Z + ZRthe Bud

TBb 485 656SAB 53 418MB 50 324BB 68 816

a The values used to calculate the ratios are given by Figures 1 to3, expressed as femtomoles per organ. b The values were ob-tained from 240 terminal buds (TB) or subapical (SAB), median (MB),and basal (BB) lateral buds.

cence of the buds while the pattern of changes in ABA levelswas not clear.

In the case of the relation between IAA and CKs of the Z-family, the calculated ratios were distributed according to an

apico-basal gradient and appeared consistent with growingcapabilities of the buds: there was a clear decrease withincreasing age, except for the oldest basal buds. There was a

discrepancy between bud activity and this ratio: the highestratio corresponding to the lowest absolute values was foundin the oldest fully inhibited buds.

DISCUSSION

The aim of this investigation was to delineate the role ofplant hormones on the growth arrest of lateral buds by the

main shoot apex. To our knowledge, this report seems to bethe first determining the contents of the main classes of freeendogenous hormones in relation to the inhibition of axillarybud growth by the shoot tip in fern sporophytes. In the waterfern on which we worked, some internal factors such as thelevels of endogenous hormones and their distribution withinthe plant are more important than the nutritive and environ-mental influences, all the buds being immersed in the samenutrient solution. Thus, there is good reason to comparegrowing and quiescent buds to demonstrate the possible roleof these regulators. We chose to quantify their levels withinbuds of intact plants because all the experiments we made ondecapitated plants were not able to explain the mechanism ofapical dominance (2, 3, 16). They showed only the privilegedstatus of the subapical bud and the rapidity of the eventsoccurring in it (14).As already emphasized by Horemans et al. (6), it is some-

times difficult to choose the appropriate basis on which growthregulator concentrations can be expressed. We chose to ex-press the hormone contents in terms of per bud, whichappeared appropriate for nongrowing lateral buds built upwith the same tissues: apex stricto sensu, leaf, and root pri-mordia, and the bud axis (12, 15) precisely cut off as noticedin "Materials and Methods." But we also gave the contentson the basis of DW more suitable for the growing terminalbud.Many reports support the fundamental role of naturally

occurring IAA on apical dominance: its synthesis and its polartransport are the clues about the mechanism of its control (4,5, 21). Our data have clearly indicated that not only IAA, butalso CKs, played a key role in the regulation of bud growth.Variations in their contents, with very rare exceptions, werewell correlated to variations in growth inhibition.The most striking feature was that the concentrations of

free IAA in the actively growing shoot apex were higher thanthose in inhibited lateral buds where they decreased about 3-fold with age according to a very evident concentration gra-dient, and with a nearly 10-fold drop if expressed per bud.CKs of the Z-type traced a pattern almost identical to that ofIAA, i.e. decreasing within the inhibited buds and peaking inthe terminal bud when represented on the per organ basis:inhibited buds contained 6- to 10-fold less CKs of the Z typethan the terminal bud. Here again levels appeared to be afunction of bud age: the lowest concentrations were found inthe median and basal buds. The relative distribution of Z +ZR per bud also showed a steep downward gradient as forIAA. We have recently shown (18) by immunocytochemicalmethods that tomato inhibited buds are well correlated withvery low contents of endogenous CKs in their quiescentmeristematic cells, and we concluded that CKs are necessaryfor axillary bud growth. Thus, as for IAA, the distribution ofZ-type CKs within the buds fits well with growth activity:actively growing buds contained the highest levels of thesehormones. Our results are also suggestive of a connectionbetween these contents and the ability of buds for furthergrowth upon decapitation. Furthermore, they can be consid-ered as physiologically important because inhibited buds wereable to respond to applications of synthetic CK-like kinetin( 11) or naturally occurring IAA (1). Therefore, in intact plants

DW

13

500

250

0

50

25

0TB SAB

910 PILATE ET AL.

https://plantphysiol.orgDownloaded on December 30, 2020. - Published by Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

ENDOGENOUS HORMONES AND APICAL DOMINANCE

ofM. drummondii, we can assume that the levels of IAA andZ-type CKs in the main shoot apex play an important role inthe correlative inhibition between buds.No such positional differences were observed with the iP-

type CKs. It is of interest that the subapical bud was found tocontain an enormous level of iPA and that the median budcontained 10 times more iPA than the terminal bud. But,there was a lack of positive correlation between iPA contentand bud age: the terminal and the basal buds were very similar.These results could explain that these buds have differentcapabilities to grow upon decapitation. The high iPA contentof the subapical bud possibly pointed out its ability for rapidand continuous growth, whereas the median bud starts grow-ing briefly and then stops. The basal buds, with their very lowcontent of iP-type CKs, are the first to be quiescent again.We do not know what happened to the high iPA content inthe subapical bud when it became a median and then a basalbud. Possibly, it was transported to the terminal bud actingas a strong sink (3, 13) where it was rapidly converted to themore active Z-type CKs, since in this actively growing bud,the iP-type CK content was low. This result supports thehypothesis that the subapical and median buds, deficient in aspecific enzymatic step, were unable to hydroxylate iP to Z,but could accumulate iPA. They had the capacity to storesufficient CK, as observed in some higher plants (23, 24), toassure their rapid outgrowth upon decapitation, the rapidgrowth arrest of the basal buds being due to the fact they haveno iP-type CKs to convert to Z-type CKs. Interestingly, recentworks on intact plants of Pisum sativum (7, 10) showed thatzeatin, but not iPA, is very efficient to release axillary budsfrom the apical control.On the contrary, we found no clear relationship between

bud inhibition and ABA levels. The immunoassays revealedthe presence of almost equal amounts ofABA within all butthe subapical buds. It was difficult to be sure that this regulatorpresent at higher concentration in the subapical bud played akey role in the control of its starting inhibition: on a per organbasis, its content was equal to that of the actively growingshoot apex. It is worthy to note that ABA, classified as aninhibitor counteracting the effects of growth promoters, wasin higher proportion relative to IAA in nongrowing buds.Thus, the absolute values were less interesting with regard tothe bud growth potentials than the relative contents. We areaware, of course, that the control of a growth process by amolecule is not only dependent on its concentration, but alsoon a change in sensitivity of the tissues to this moleculeaccording to Trewavas (22). We know also that its compart-mentation in the buds depends on their age and affectsselected tissues of these structurally heterogeneous organs (17,19). Therefore, it seems important to analyze the subcellularlocalization of the hormone. This important question can beanswered by the use of immunocytochemical methods tolocalize plant hormones as initiated in our laboratory (17-19)to delineate the possible role of a regulator not based on itstissue or organ concentration.

Significant differences in the IAA and Z-type CK levelsmay explain the apical control of the main shoot apex on thelateral buds it has initiated. On the contrary, there is nocorrelation between ABA content and bud inhibition. Inter-

estingly, the subapical bud precedence and, to a lesser extent,the following activation of the median bud in decapitatedplants could be related to their high iPA content playing therole of a storage CK form.

ACKNOWLEDGMENT

We wish to thank Dr. Regis Maldiney for computer graphics andhis helpful assistance in the manuscript preparation.

LITERATURE CITED1. Chenou-Fleury E (1975) Recherches sur le determinisme de la

dominance apicale chez la Fougere aquatique Marsilea drum-mondii: apport d'observations histologiques des bourgeons la-teraux de sporophytes intacts et decapites. Rev Cyt Biol Veg38: 111-196

2. Chenou E, Sossountzov L (1982) Influence des blessures meca-niques sur la croissance des bourgeons inhibes du Marsileadrummondii A Br. Beitr Biol Pflanzen 56: 117-132

3. Chenou E, Sossountzov L, Lefebvre MF (1978) Distribution deI'AIA- 14C en relation avec l'inhibition de croissance des bour-geons: etude d'une Fougere, le Marsilea drummondii A. Br.Physiol Veg 16: 137-156

4. Hillman JR (1986) Apical dominance and correlations by hor-mones. In M Bopp, ed, Plant Growth Substances 1985. Sprin-ger-Verlag, Berlin, pp 341-349

5. Hillman JR, Math VB, Medlow GC (1977) Apical dominanceand the levels of indole acetic acid in Phaseolus lateral buds.Planta 134: 191-193

6. Horemans S, Van Onckelen HA, De Greef JA (1986) Longitu-dinal gradients of indole-3-acetic acid and abscisic acid inhypocotyl of etiolated bean seedlings. J Exp Bot 37: 1525-1532

7. King RA, Van Staden J (1988) Differential responses of budsalong the shoot of Pisum sativum to isopentenyladenine andzeatin application. Plant Physiol Biochem 26: 253-259

8. Leroux B, Maldiney R, Miginiac E, Sossountzov L, Sotta B(1985) Comparative quantitation of abscisic acid in plantextracts by gas-liquid chromatography and an enzyme-linkedimmunosorbent assay using the avidin-biotin system. Planta166: 524-529

9. Maldiney R, Leroux B, Sabbagh I, Sotta B, Sossountzov L,Miginiac E (1986) A biotin-avidin-based enzyme immunoas-say to quantify three phytohormones: auxin, abscisic acid andzeatin-riboside. J Immunol Methods 90: 151-158

10. Pillay I, Railton ID (1983) Complete release of axillary budsfrom apical dominance in intact, light-grown seedlings ofPisum sativum L. following a single application of cytokinin.Plant Physiol 71: 972-974

11. Sossountzov L (1967) Etude morphologique et histologique dusporophyte de la fougere aquatique Marsilea drummondii. A.Br. cultivee in vitro: structure et fonctionnement de l'apex.IIle sporophyte traite par la kinetine. Rev Cyt Biol Veg 30: 1-190

12. Sossountzov L (1969) Incorporation de precurseurs trities desacides nucleiques dans les meristemes apicaux du sporophytede la fougere aquatique, Marsilea drummondii A Br. Rev GenBot76: 106-156

13. Sossountzov L, Chenou E (1978) Transport et metabolisme de 1'AIA- 14C ou de l'AIA-3H dans les bourgeons lateraux d'unefougere, le Marsilea drummondii A Br. Physiol Veg 16: 617-641

14. Sossountzov L, Bomsel JL, Vinel D, Chenou E (1982) Analysisof apical dominance in the aquatic fern, Marsilea drummondiiA Br: Evolution of energy metabolism in lateral buds afterdecapitation of the sporophyte. J Plant Growth Regul 1: 173-187

15. Sossountzov L, Habricot Y (1985) Ultracytochemical localizationand characterization of membrane-bound ATPases in lateralbuds from intact and decapitated plants of an aquatic fern,Marsilea drummondii A Br. Protoplasma 127: 180-191

911

https://plantphysiol.orgDownloaded on December 30, 2020. - Published by Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

912 PILATE ET AL.

16. Sossountzov L, Habricot Y, Garrec JP, Lamant A (1985) Earlyeffects of decapitation on the Mg2+-K' ATPase and cationcontents in lateral buds of the aquatic fern, Marsilea drum-mondii A Br. Protoplasma 127: 192-203

17. Sossountzov L, Sotta B, Maldiney R, Sabbagh I, Miginiac E(1986) Immunoelectron-microscopy localization of abscisicacid with colloidal gold on Lowicryl-embedded tissues of Chen-opodium polyspermum L. Planta 16: 471-481

18. Sossountzov L, Maldiney R, Sotta B, Sabbagh I, Habricot Y,Bonnet M, Miginiac E (1988) Immunocytochemical localiza-tion of cytokinins in Craigella tomato and a sideshootlessmutant. Planta 175: 291-304

19. Sotta B, Sossountzov L, Maldiney R, Sabbagh I, Tachon P,Miginiac E (1985) Abscisic acid localization by light micro-scopic immunohistochemistry in Chenopodium polyspermumL. Effect of water stress. J Histochem Cytochem 33: 201-208

Plant Physiol. Vol. 90, 1989

20. Sotta B, Pilate G, Pelese F, Sabbagh I, Bonnet M, Maldiney R(1987) An avidin-biotin solid phase ELISA for femtomoleisopentenyladenine and isopentenyladenosine measurementsin HPLC purified plant extracts. Plant Physiol 84: 571-573

21. Tamas IA (1987) Hormonal regulation of apical dominance. InPJ Davies, ed, Plant Hormones and Their Role in PlantGrowth Development. Martinus Nijhoff, Dordrecht, pp 132-148

22. Trewavas AJ (1982) Growth substance sensitivity: the limitingfactor in plant development. Physiol Plant 55: 60-72

23. Tsui C, Shao LM, Wang CM, Tao GQ, Letham DS, Parker CW,Summons RE, Hocart CH (1983) Identification of a cytokininin water chestnuts (corms of Eleocharis tuberosa). Plant SciLett 32: 225-231

24. Wang TL, Wareing PF (1979) Cytokinins and apical dominancein Solanum andigena: lateral shoot growth and endogenouscytokinin levels in the absence of roots. New Phytol 82: 19-28

https://plantphysiol.orgDownloaded on December 30, 2020. - Published by Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.